The present invention relates to wire bonders in general and, more particularly, to tailless wire bonders.
Heretofore, tailless wire bonders have suffered from various deficiencies limiting their performance or requiring complex and costly mechanisms to circumvent the problems. One such machine which is described in U.S. Pat. No. 3,643,321, overcomes some of the problems encountered in earlier machines by a mechanism which clamps the wire during the second bond, raises the bonding tool a predetermined distance while the clamping device remains stationary, and then raises both the bonding tool and the clamping device together to break the wire at the bond leaving a tailless bond and a predetermined length of wire protruding below the bonding tool. Subsequently the bonding tool & clamping device rise to a fixed position and a torch is moved close to the protruding wire to melt the end of the wire forming a small spherical ball.
It is most important to the subsequent bonding step that this ball is consistent in size. An important factor in controlling the ball size is the length of wire protruding below the bonding tool and the location of the end of the wire relative to the position of the torch. Precise control of these variables in the referenced machine requires that the final bonding operation take place at a fixed and predetermined position along the path of motion of the bonding tool. A machine adjustment must be provided for this purpose and extreme care must be taken to hold the workpiece at the correct position.
The present invention provides a means for achieving a tailless final bond whereby the bonding operation may take place at any location along the path of the bonding tool without affecting the length of wire protruding below the bonding tool or the location of the end of the wire relative to the torch. A substantial simplification in the workholder results from this improvement. A further advantage of the invention is that the length of wire protruding below the bonding tool is determined by only one (adjustable) member, rather than several cams and linkages, whereby a precise wire length can be achieved without requiring many precision parts.
Thermocompression wire bonders generally use some means of providing a controlled amount of drag or tension in the wire feeding into the bonding tool in order to control the size and shape of the wire loop between the first and final bonds. Some machines use a simple friction pad squeezing the wire. Others use a combination of a friction pad and a spring to maintain tension in the wire in case the bonding tool is moved in a backwards direction tending to shorten the wire. It is desireable to maintain constant tension in the wire during formation of the loop regardless of the motion of the bonding tool and to be able to adjust the tension conveniently to the desired amount.
Devices employing friction are notoriously variable in their characteristics and generally unreliable for producing small, precise forces. Furthermore, spring devices heretofore used are unable to provide an adjustable force that then remains constant over a reasonably large deflection. The present invention overcomes both of these difficulties providing a constant and adjustable tension without the use of frictional devices.
Wire bonders used in the semiconductor industry generally use manual positioning by an operator of the workpiece relative to the bonding tool. The machines are provided with various devices to assist the operator such as a micropositioner, a viewing stereo microscope and in some cases an optical device, (such as shown in U.S. Pat. No. 3,661,316) projecting a spot of light onto the workpiece at the location where the bonding tool will make contact. The optical spot projector suffers from the defects of parallax error since the projected beam of light must be several degrees from vertical, low spot brightness, a rather large, bulky structure and a shimmering instability of the spot position caused by the heated air rising from the workpiece. The present invention provides an optical positioning aid of comparable effectiveness but free of all of these deficiencies.
It is a general object of the present invention to provide an improved tailless wire bonder.
It is a specific object of the invention to provide a tailless, thermocompression wire bonder which permits wire bonding at any location along the path of the bonding tool.
It is another specific object of the invention to provide a tensioning device for thermocompression wire bonders which produces a constant and adjustable tension without utilizing frictional means.
It is still another object of the invention to provide an improved optical positioning apparatus for wire bonders.